1. Melt butter in a large skillet. Add onions; cook for 20 minutes, stirring frequently, over medium-low heat until very soft and golden brown.

2. Meanwhile, heat lager and French dip sauce in a small saucepan until hot. Dip pastrami slices into hot beer mixture and pile onto rolls; top with equal amounts of caramelized onions and close sandwiches. Serve remaining beer mixture on the side with a small ladle or spoon.

Tip: You can reduce the fat in this recipe by opting for turkey pastrami.